Simplify CSS, Improve Inline Form Styling, Make Admin.css More Generic

I went through the CSS rules in admin.css and tried to make them as generic and as simple as possible.

Here's are some notes about the changes:
* Move "body {padding-top: 4px;}" into base.html template, because it's styling specifically for that base template.
* Added more documentation to existing CSS rules.
* Added more specific classes to some CSS rules, to prevent accidentally styling things unintentionally.
* Improves styling for inline forms for both bootstrap 2 and 3.
 Before:
![inline_form_before](https://cloud.githubusercontent.com/assets/992533/5563362/28b1ebde-8e32-11e4-80a6-b23ba9635eb4.png)

 After:
![inline_form_after](https://cloud.githubusercontent.com/assets/992533/5563357/0e5ea574-8e32-11e4-927c-b183c6f6d636.png)

* Moves bootstrap3's help-block to where it's supposed to be.
 Before:
![help_block_before](https://cloud.githubusercontent.com/assets/992533/5563380/a1354eac-8e32-11e4-8094-cb8f3e7b31e4.png)

 After:
![help_block_after](https://cloud.githubusercontent.com/assets/992533/5563381/a4d56786-8e32-11e4-910c-dda02da10a1f.png)

* Remove checkbox padding for bootstrap 3.
 Before:
![after_check](https://cloud.githubusercontent.com/assets/992533/5563389/329b4766-8e33-11e4-9487-d3f370776877.png)

 After:
![before_check](https://cloud.githubusercontent.com/assets/992533/5563388/2ee06340-8e33-11e4-9582-4439a7444e5e.png)

* Use bootstrap 3 classes to add "clear search" button to search.
 After:
![after_append](https://cloud.githubusercontent.com/assets/992533/5563392/96fecbec-8e33-11e4-8872-0f1e49238b6b.png)

* Removed hardcoded color on the trash can icon for bootstrap 3. Instead, prevent the <a> element's color from being applied.
* Removed btn-cancel and btn-link for bootstrap 3 and just used bootstrap defaults.